It is known that the resonance frequency of a piezoelectric resonator can be shifted by the addition of a load capacitor in series with the resonator. It is also known that the frequency of a piezoelectric resonator will vary with temperature. The frequency excursion as a function of temperature depends on both the material properties of the resonator and the value of the load capacitor placed in series with the resonator. This dependence has been described in the paper "Frequency-Temperature-Load Capacitance Behavior of Resonators for TCXO Application" by Arthur Ballato as published in the IEEE Transactions On Sonics and Ultrasonics, Vol. SU-25, No. 4, July 1978.
Numerous schemes have been devised in the past to compensate for the temperature sensitivity of the resonator. These have generally involved measuring the temperature excursion of the resonator at a fixed value of load capacitance, and the synthesizing of a circuit wherein the value of load capacitance is varied in such a manner as to compensate for the temperature induced frequency excursion in the resonator. These schemes, although capable of producing oscillators with .+-.1 part per million frequency stability over operating temperature range when new, are typically found lacking after a period of time has elapsed and trim adjustments have been made to compensate for aging. This is because the trim adjustments to compensate for aging are accomplished by changing the value of series load capacitance by an additional fixed amount independent of temperature, which has both the desired effect of adjusting the frequency at the temperature at which the adjustment is made and the undesired effect of changing the frequency-temperature behavior of the resonator. Since a TCXO is designed to compensate for the frequency temperature behavior at a chosen value of load capacitance, the compensation is degraded when the load capacitance is adjusted to a different value.